The long term objective of this proposal is to elucidate the host-defense mechanisms against bovine leukemia virus (BLV) infection, pre-neoplasia and neoplastic progression of the disease by exploiting the genetic differences between animals for these traits. Recent findings in our laboratory demonstrate the intimate involvement of bovine major histocompatibility complex (MHC) class II genes in resistance and susceptibility to the development of persistent lymphocytosis (PL) in BLV-infected cattle. The hypothesis to be addressed in this proposal is that MHC-linked control of helper T cell responses to BLV peptides is the underlying mechanism of resistance to the spread of BLV infection to multiple target cells in vivo. To test this hypothesis we propose a series of experiments with the following specific aims: I. Determine whether alleles of the bovine major histocompatibility complex DRB3 locus (BoLA-DRB3) affect the spread of BLV infection within naturally infected hosts. II. Evaluate BoLA-DRB3-linked control of helper T cell responses to the BLV polymerase peptide HVFVDTYSGATH in naturally infected cattle. III. Identify additional immunodominant peptides of BLV gag, pol, env, tax and rex proteins and evaluate the genetic control of T cell responses to these peptides in naturally infected cattle. Our studies will employ MHC-defined, naturally exposed cattle to test hypotheses that relate MHC genotype to: a) changes in the number of BLV- infected cells in peripheral blood (Aim I); and b) differences in T cell responses to BLV peptides (Aims II & III). We will accomplish Aim I using state-of-the-art technology that we have recently developed in our laboratory; the diagnosis of BLV provirus in single cells using the polymerase chain reaction (PCR) to amplify virus-specific sequences in fluorescently labeled, flow-sorted leukocytes. Using this technique, we will determine the frequency and absolute numbers of BLV-infected B cells, T cells and monocytes in MHC-defined, BLV-infected cattle with and without PL. Furthermore, we will determine whether bone marrow-derived macrophages (BMDM) are permissive for virus expression using in vitro- generated colonies of BMDM and a definitive reverse transcription (RT)- PCR test for BLV expression in these cells. The genetic control of T cell responses to BLV peptides will be determined by measuring IL-2 responses to synthetic peptides in vitro using the bovine IL-2 dependent cell line, BT2. Definition of such mechanisms may provide valuable insights into why HIV-infected humans show variation in the time until seroconversion and the development of ARC and AIDS, and why there is an increased frequency of neoplasia associated with HIV infection.